1. Technical Field of the Invention
The present invention relates generally to an automatic brake control system for vehicles, and more particularly to an improved hydraulic circuit structure of a brake control system designed to regulate the braking effort applied to each wheel both in manual and automatic control modes.
2. Background Art
Japanese Patent First Publication No. 8-230634 teaches a brake control system which uses a brake assisting device in a hydraulic circuit extending from a master cylinder to wheel cylinders in order to eliminate the need for a vacuum brake booster and enhance the braking effort applied to each wheel upon urgent braking. This system is also capable of producing the wheel cylinder pressure higher than the master cylinder pressure to shorten the braking distance.
Such a brake control system usually has pumps, one in each hydraulic line, to provide the hydraulic pressure higher than the master cylinder pressure to wheel cylinders. In a case where a vehicle is equipped with two hydraulic lines: one being a front hydraulic line for front right and left wheels and the second being a rear hydraulic line for rear right and left wheels, if a difference in output between pumps disposed in the front and rear hydraulic lines is developed, it will result in a difference in braking effort applied to the front and rear wheels, but do not cause a vehicle body to undergo yawing. However, in a case where a vehicle is equipped with a diagonal hydraulic system (usually used in FF (front engine front drive) vehicles) consisting of a first hydraulic line for front right and rear left wheels and a second hydraulic line for front left and rear right wheels, if a difference in output between pumps disposed in the first and second hydraulic lines is developed, it will cause a vehicle body to yaw, which may result in instability of the braking.